Long-form guides for homeowners replacing a thermostat: how the 24 V circuits work, what each wire does, and how to solve the common problems. Written from the manufacturers’ published installation manuals.
How to wire a Honeywell Home RTH6360D from Resideo’s install guide: the UWP terminals, the R-switch that replaces jumpers, why it needs batteries even with a C wire, heat pump wiring with AUX and E, and setup options 200 to 221.
How to wire a Honeywell Home T4 Pro (TH4110U2005 or TH4210U2002) from Resideo’s install guide: which UWP terminals are used, the R slider tab, batteries vs C wire, heat pump O/B setting 218, and why W stays empty on a heat pump.
How to wire a Honeywell RTH2300 or RTH2300B from Honeywell’s install guide: the six terminals, the R–RC jumper, JP1 and JP2 settings, heat pump wiring with the Y–W jumper, and why the C wire is not used.
How to wire a Honeywell Home T10 Pro from Resideo’s install guide: every UWP terminal, the required C wire and K adapter, the R slider tab, and why heat pump aux heat goes to Aux or E, never W.
How to wire an ecobee3 lite from ecobee’s install guide: the 10 terminals, where one or two R wires go, the PEK moves without a C wire, and heat pump O/B and aux wiring.
How to wire a Honeywell RTH6580WF from its user guide: each screw terminal, the required C wire, the R-RC jumper, the unused K terminal, and heat pump function 1 and 2 setup.
The ecobee Smart Thermostat Premium wiring diagrams in plain text, from ecobee’s manual: each terminal, the PEK+ moves without a C wire, and heat pump aux wiring.
How to wire the Amazon Smart Thermostat from Amazon’s help pages: the required C wire, where G and Y move with the Honeywell adapter, the R switch and heat pump O/B setup.
How to wire a Honeywell RTH9585WF from its user guide: where each wire goes, the shared W-O/B terminal, the required C wire, the K terminal adapter and heat pump setup.
How to wire a Wyze Thermostat from Wyze’s support articles: why it always needs a C wire, fitting the C-Wire Adapter at the control board, three-wire systems and heat pumps.
How to wire the mirror-finish Google Nest Thermostat from Google’s help pages: each connector, the *OB star terminal, Rh and Rc, heat pumps and when you need a C wire or Power Connector.
How to wire a Sensi thermostat from Copeland’s install guide: which models and systems need a C wire, the built-in Rc/Rh jumper, both label sets and the O/B setting.
How to wire a Honeywell T5+ thermostat from Honeywell’s manual: the R-switch for one or two R wires, terminal functions, jumper removal, C-wire options and heat pump ISU settings.
Which wire goes on O/B, Aux, E and C on a Honeywell T6 Pro heat pump install, why W stays empty, and the ISU 200, 218, 220 and 221 settings that match.
What Nest help codes E74, E24, E4, E73 and E195 mean, why the thermostat sees no power on Rh, Rc or R, and the checks that fix it, from the breaker to the control-board fuse.
What Cool On or Heat On flashing, Wait, In Recovery, temporary and permanent hold, Aux Heat, Delayed, Calibrating, No power to Rh, thermostat icons and furnace blink codes mean, and what to do.
What temperature to set in winter and summer, at night and when away, fan Auto vs On, heat pump setbacks, humidity, hold and schedules, where to put a thermostat and how to reset one.
Diagrams for 2, 3, 4, 5, 6, 7 and 8-wire thermostats, furnace and AC, heat pumps and boilers, the wire color code, and where each old wire goes on Nest, ecobee, Honeywell and Amazon.
O vs B reversing valves, auxiliary and emergency heat, what happens during defrost, why dual-fuel systems must never run both at once, and how to choose a switchover temperature.
How the 24 V control circuit works, why wire colors can’t be trusted, what power stealing does, four ways to get a C wire, and the meter checks that find most problems.
